Presently in RFID tagging, there is interest in inexpensive item-level tags that require microradio chips, or microradios, coupled in some manner to an associated antenna. The reason for the utilization of these microradio chips and their associated antennas is that the major cost of the RFID tag is embodied in the integrated circuit chip. By making the RFID chips smaller, one can cost-effectively mass-produce them by fabricating millions of microradio chips on a single semiconductor wafer.
Conventionally, the problem with RFID chips is the cost associated with mounting and electrically connecting them to the feed point of the antenna so that RP energy may be effectively coupled from the antenna to the RFID chip and vice versa.
As described in U.S. Application Ser. No. 60/711,217 filed Aug. 25, 2005 by Kenneth R. Erickson, one of the ways to connect the RFID tag electronics to its associated antenna is to apply a multitude of microradio chips, suspended in a slurry, at the vicinity of the feed port of the antenna. In one approach to the coupling of the RFID electronics to the antenna, each microradio chip is provided with opposed electrically conductive end pieces or tabs, one of which is embedded in a conductive ink trace for one side of the antenna, whereas an opposing electrically conductive tab is embedded in an overlying patterned conductive trace for the other side of the antenna. In this way the RFID chips may be directly DC coupled to the antenna at its feed points.
Because of the large number of microradio chips contained in the slurry, it is indeed probable that at least one of them will be oriented appropriately so as to connect one of the conductive tabs to one portion of the antenna at its feed point and the tab at the other end of the microradio chip to the overlying conductive stripe that is connected to the portion of the antenna.
While it is recognized that direct DC coupling is a more robust way of coupling RF energy into and out of the microradio chip, it is also possible to provide a non-DC contact electromagnetic coupling between a collection of microradio chips in suspension within a slurry and the associated antenna feed points.
Regardless of whether the microradio chips have opposing conductive contacts, as required for the direct DC-couple case, or whether they couple electromagnetically, each will naturally exhibit a polarity. In the electromagnetic coupling case, the microradio chip will form an electric field dipole regardless of the antenna topology, be it a slot, a dipole, a patch, or a loop, employed in its design and construction. This electric field dipole, and hence the microradio chip, has a polarity depending upon which way the signal generator within the chip is connected to its internal antenna's feed port.
In both the direct DC-coupled and the electromagnetically-coupled cases, when many of these microradio chips are utilized to couple to a single RF tag antenna, there needs to be a method to ensure that they operate coherently, that is that their respective contributions add constructively rather than canceling each other out. The orientation, and hence the polarization, of microradios suspended within the slurry will tend to be random, statistically resulting in a significant degree of signal cancellation. In the direct DC-coupled case, for example, roughly half of the microradio chips contacting the tag antenna leads will have a polarization that opposes that of the other half.
It is important to be able to reverse the polarity of all of those suspended microradio chips that are oriented in one of these two polarization states so that all chips contributing to the RFID function electrically point in the same direction and so that their outputs add constructively.
Put another way, with the fluid suspension of the small electromagnetically-coupled microradio chips, each of these microradios has an associated electrical orientation because of the way it radiates through its electric dipole structure. Thus, when viewing the microradio chip, it is appropriate to say that one side of the chip has an electrical “north” and the opposite side has an electrical “south”. When thousands of these microradio chips are suspended in a fluid and deposited in the vicinity of the feed port of the tag antenna, their north ends will tend to be in a random physical orientation relative to each other. Some will have their north ends closer to the upper tag antenna contact, while others will have their north ends closer to the lower tag antenna contact.
There therefore needs to be a way to first select the microradio chips whose north-south orientation is such that a significant portion of their radiating effects will contribute to the excitation of the tag antenna Secondly, there needs to be a way to selectively reverse the north-south polarization, so that when they radiate they will all radiate coherently with the other microradio chips in the suspension.
When these microradio chips are probed by an REID tag reader, all of them transmit simultaneously. If they are not oriented substantially parallel to each other, then it is possible that the radiation from one of the microradio chips would cancel out the other. Thus it is a requirement that one activate only those micro RFID devices that have a roughly similar physical orientation (i.e. north-south axis within say 30° of being perpendicular to the planes of the RF tag antenna leads) and among those, it is a requirement that the polarization direction be set so that all north axes align.